title: Unusual effect of water vapor pressure on dehydration of dibasic calcium phosphate dihydrate.
abstract: Dibasic calcium phosphate occurs as an anhydrate (DCPA; CaHPO₄) and as a dihydrate (DCPD; CaHPO₄•2H₂O). Our objective was to investigate the unusual behavior of these phases. Dibasic calcium phosphate dihydrate was dehydrated in a (i) differential scanning calorimeter (DSC) in different pan configurations; (ii) variable-temperature X-ray diffractometer (XRD) at atmospheric and under reduced pressure, and in sealed capillaries; and (iii) water vapor sorption analyzer at varying temperature and humidity conditions. Dehydration was complete by 210°C in an open DSC pan and under atmospheric pressure in the XRD. Unlike "conventional" hydrates, the dehydration of DCPD was facilitated in the presence of water vapor. Variable-temperature XRD in a sealed capillary and DSC in a hermetic pan with pinhole caused complete dehydration by 100°C and 140°C, respectively. Under reduced pressure, conversion to the anhydrate was incomplete even at 300°C. The increase in dehydration rate with increase in water vapor pressure has been explained by the Smith-Topley effect. Under "dry" conditions, a coating of poorly crystalline product is believed to form on the surface of particles and act as a barrier to further dehydration. However, in the presence of water vapor, recrystallization occurs, creating cracks and channels and facilitating continued dehydration.
authors.affiliation: Department of Pharmaceutics, College of Pharmacy, University of Minnesota, Minneapolis, Minnesota 55455.
authors.forename: Aditya M
authors.initials: AM
authors.lastname: Kaushal
authors.forename: Venu R
authors.initials: VR
authors.lastname: Vangala
authors.forename: Raj
authors.initials: R
authors.lastname: Suryanarayanan
mesh_terms: D002130:Calcium Phosphates / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D013227:Steam / Q000032:analysis*; D013818:Thermogravimetry; D055548:Vapor Pressure; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D002130:Calcium Phosphates; D013227:Steam; D014867:Water; C485829:calcium phosphate, dibasic, anhydrous
keywords: X-ray diffractometry; calorimetry (DSC); dehydration; dibasic calcium phosphate; excipients; hydrate; physical stability; thermogravimetric analysis; water vapor